The present invention relates to a defect inspecting apparatus for inspecting a defect on the surface of an object to be inspected, such as a semiconductor wafer.
As a method of inspecting a defect such as a scar or dust on the surface of a semiconductor wafer (hereafter also referred to as the wafer), a method is known which makes use of dark field illumination in which illuminating light is applied to the wafer surface from a diagonal direction.
By making use of this dark field illumination, a defect is conventionally inspected by an operator by rotating or tilting the wafer and by visually observing the presence of an abnormal spot. In recent years, however, to automate the inspection, a method has been proposed in which the overall wafer is imaged by a CCD camera, and its image data is subjected to image processing so as to inspect a defect.
However, since the scattered light from a defect such as a scar or dust on the basis of dark field illumination is very weak, portions other than the defect should preferably be relatively dark in performing inspection based on image processing. However, in a wafer having a very small periodic pattern formed thereon, diffracted light occurs due to the pattern, so that it is difficult to create a dark field state for inspecting the defect with high sensitivity. Further, the arrangement becomes complicated in realizing by image processing those which have been done by the operator, and there is a problem in the processing speed as well.
Further, an operation for examining and setting in advance an optimum illuminating angle is required in image processing. However, since an optimum illuminating angle differs depending on the type of wafer, much time and trouble is required to cope with a large number of types of wafer. Furthermore, in the case of a wafer having a pattern for which a sufficient dark field state cannot be obtained at whatever illuminating angles, the defect detection sensitivity becomes poor.
In view of the problems of the above-described conventional art, it is an object of the present invention to provide a defect inspecting apparatus which is capable of effecting defect inspection easily with high sensitivity without performing complicated adjustment or processing even with respect to objects to be inspected having different types of periodic patterns.
(1) A defect inspecting apparatus for inspecting a defect of an object having a periodic pattern, said apparatus comprising:
a dark field illumination optical system that illuminates the object with substantially parallel illumination light in a direction having a predetermined first inclined angle relative to an inspection surface of the object;
an imaging optical system having an imaging element for imaging the object illuminated with the illumination light, the imaging element having an imaging lens; and
defect detecting means for detecting the defect based on image data of the object thus imaged;
wherein a mutual positional relationship between the direction of illumination by the illumination optical system and a direction of imaging by the imaging optical system is determined based on a diffraction angle defined by a period of the pattern and a wavelength of the illumination light; and
wherein a sum of an aperture angle of the illumination light and an object side aperture angle of the imaging lens is set to be smaller than the diffraction angle.
(2) A defect inspecting apparatus of (1), wherein the imaging optical system is disposed to image the object in an angular direction falling within a diffraction angle defined between zero-order diffraction light and 1st-order diffraction light both being caused due to presence of the pattern.
(3) A defect inspecting apparatus of (1), wherein the imaging optical system is disposed to image the object in an angular direction located substantially at a middle of a diffraction angle defined between zero-order diffraction light and 1st-order diffraction light both being caused due to presence of the pattern.
(4) A defect inspecting apparatus of (2), wherein the illumination optical system is disposed to emit the illumination light in the direction that has the predetermined first inclined angle relative to the inspection surface of the object and that perpendicularly intersects a periodic direction of the pattern.
(5) A defect inspecting apparatus of (2), wherein:
the imaging optical system is disposed to image the object in a direction substantially perpendicular to the inspection surface of the object; and
the illumination optical system is disposed to emit the illumination light in the direction that has the predetermined first inclined angle relative to the inspection surface of the object and that has a predetermined second inclined angle relative to a direction perpendicularly intersecting a periodic direction of the pattern.
(6) A defect inspecting apparatus of (5), wherein the second inclined angle is substantially identical to an angle defined between a direction of the zero-order diffraction light or the 1st order diffraction light and the imaging direction.
(7) A defect inspecting apparatus of (1), wherein the mutual positional relationship between the illuminating direction and the imaging direction is determined based on a diffraction angle defined by a maximum period of the pattern and a minimum wavelength of the illumination light.
(8) A defect inspecting apparatus of (1), further comprising:
means for mutually varying the illuminating direction relative to the object;
image preparing means for extracting pixel data of a smallest luminance from data of positionally identical pixels on images of the object taken each time when the illuminating direction is varied, and preparing image data of the object from the extracted pixel data,
wherein the defect detecting means detects the defect based on the thus prepared image data.
(9) A defect inspecting apparatus of (8), wherein the varying means includes rotating means for rotating the dark field illumination optical system relative to the object.
(10) A defect inspecting apparatus of (8), wherein the varying means includes rotating means for rotating the object and the imaging optical system.
(11) A defect inspecting apparatus of (8), wherein the dark filed illumination optical system or the imaging optical system has means for setting a wavelength band to a narrow band.
(12) A defect inspecting apparatus for inspecting a defect of an object having a periodic pattern, said apparatus comprising:
a dark field illumination optical system that illuminates the object with substantially parallel illumination light in a direction having a predetermined inclined angle relative to an inspection surface of the object;
an imaging optical system having an imaging element for imaging the object illuminated with the illumination light, the imaging element having an imaging lens;
means for mutually varying the illuminating direction relative to the object;
image preparing means for extracting pixel data of a smallest luminance from data of positionally identical pixels on images of the object taken each time when the illuminating direction is varied, and preparing image data of the object from the extracted pixel data,
defect detecting means for detecting the defect based on the thus prepared image data;
wherein a sum of an aperture angle of the illumination light and an object side aperture angle of the imaging lens is set to be smaller than a diffraction angle defined by a period of the pattern and a wavelength of the illumination light.
(13) A defect inspecting apparatus of (12), wherein the varying means includes rotating means for rotating the dark field illumination optical system relative to the object.
(14) A defect inspecting apparatus of (12), wherein the varying means includes rotating means for rotating the object and the imaging optical system.
(15) A defect inspecting apparatus of (12), wherein the dark filed illumination optical system or the imaging optical system has means for setting a wavelength band to a narrow band.
The present disclosure relates to the subject matter contained in Japanese patent application No. Hei. 11-184881 (filed on Jun. 30, 1999), which is expressly incorporated herein by reference in its entirety.